1. Field of the Invention
This invention relates to decoders for Hamming encoded data in the form of a serial bit stream, successive blocks of which are Hamming encoded. The invention also relates to a method of decoding Hamming encoded data. The invention is particularly applicable to such decoders for use in the reception of teletext information.
2. Description of the Prior Art
The document "World System Teletext and Data Broadcasting System-Technical Specification", December 1987, compiled by the Department of Trade and Industry of the U.K. Government, discloses a system for transmitting teletext information in 625-line and 525-line television systems.
This specification, amongst other things, specifies the use of so-called EXTENSION packets and some of these packets, but particularly so-called "packet 26" "packet 28", "packet 29" and some versions of "packet 27" has coded data encoded using the 24/18 Hamming coding scheme where each 18 data bits are protected by 6 protection bits. This form of Hamming coding allows a single bit error to be detected and corrected and 2 bit errors to be detected within the group of 24 bits. Such extension packets are used to help control the teletext decoder or to supply additional information so that superior decoders can enhance the appearance of the basic display page. The enhancements may take the form of accented characters, improved graphics, etc. It is therefore necessary in a teletext decoder of a teletext receiver to provide a means of decoding the 24/18 Hamming protected data.
Up to the present time, the decoding of some of the Hamming protected data has been carried out in the teletext decoder by a microprocessor using software techniques, but this places great demands on the microprocessor and compromises the performance of the decoder. It would be advantageous if the decoding could be carried out using suitable hardware which could be incorporated on the teletext decoder integrated circuit, and thereby reduce the demand on the microprocessor.